Attempts to use modulated (e.g. pulsed) sound waves for the exploration of terrain have been quite successful in the past but did not lead to satisfactory results below ground. So-called white noise, i.e. sound waves with a broad spectrum of acoustic frequencies, gives rise to reflections at discontinuities of the soil or rock in which it is propagated, yet these reflections cannot be readily evaluated to indicate the location of such discontinuities. That type of information, however, is highly important in mining operations in order to provide clues as to the extent and distribution of coal seams to be exploited. In the absence of this knowledge, equipment may be wastefully installed in low-yield areas or may be ineffectually utilized.